known-answer tests for the T-test implementation

Add some tests for the Welch T-test code, possibly testing it against another implementation.

Maybe easiest is to use golang:
* https://pkg.go.dev/golang.org/x/perf/internal/stats#TwoSampleWelchTTest
* looks minimally tested https://cs.opensource.google/go/x/perf/+/1a17dc26:internal/stats/ttest_test.go;bpv=1;bpt=0